1. Field of the Invention
The present invention relates generally to current measurement devices, and more particularly to a current sensor using low cost current transformer arrays.
2. Description of the Related Art
Current measurements are critical to the control of magnetic resonance imaging (MRI) gradient coil signals. Conventional current sensing devices limit the dynamic range and accuracy of the images. Traditionally, current transformers/transducers or CTs have been hand selected to make a marginal product possible. A current transducer that used to cost $30 has escalated to over $300.
The best conventional current sensor is based on fluxgate design principals and costs $500 each. These sensors are plagued, however, by a substantial clock noise signal that is typically at 10 KHz and in the closed-loop bandwidth of the gradient system. It would be possible to use an external passive sensor with a fluxgate sensor. This solution, however, would cost nearly $550 per axis, and is not economically feasible.
Most galvanically isolated current sensors are based on some use of magnetics to sense the flux around the output leads of the gradient amplifier, and as such they are also susceptible to large magnetic fields that are present in the MRI environment. A typical maximum stray field specification is 200 Gauss. Unfortunately, all of the hi-mu materials used in current transducers (fluxgate included) saturate at about 50 Gauss. Clearly, whatever is used must have added shielding to reduce the external field to well under 50 Gauss near the current transducers.
The next best conventional current transducer is based on the closed-loop application of Hall sensors. A large turn-count secondary is wound onto a core through which the primary is threaded (few turns). The Hall sensor is placed into the gap of the core and its output is amplified and fed through the secondary. The secondary current is sensed with a resistor across which the output signal is produced. The feedback is poled in a way that minimizes the flux sensed by the Hall sensor. This condition occurs when the secondary current times the secondary turns count is equal to the primary current times the primary turns count. A DC coupled transformer is thus formed.
When the loop gain diminishes at high frequencies, a conventional transformer remains with the secondary current being driven into the sense resistor. The bandwidth of such a system is intrinsically large with a typical 3 dB being beyond 250 KHz.
At low frequencies, the necessary core size is minimized by the flux zeroing nature of the system. The major limitation to performance of the system is the limited signal-to-noise (S/N ) of any known Hall sensor today. The result is a sensor which exhibits at best about twice the desired noise level of the current transducer.
Virtually all current transducer manufacturers use current sensors made by Asahi Kasei Electronics (AKE) of Japan. Most of the sensors are made for high volume applications which do not have the demands of MRI current transducers. AKE does not make custom devices for current transducer manufacturers. Accordingly, such manufacturers are limited to AKE""s high volume sensors, with the option to sort them for desired performance characteristics.
Within the market of current transducers there are graduations of sensor quality and quantity. Some high volume applications have resulted in low-cost ($3-$10) current transducers of magnetic construction which is not as self-shielding as the highest performance types. The frequency response of these transducers is also more likely to contain a dip in the frequency region where the closed loop system is crossing over to the passive open loop high frequency system. Providing that the distortion and frequency response of these less expensive types can be eliminated or controlled, it would be possible to use them in a manner which results in superior stray field rejection and better S/N ratio.
Many of the lower cost current transducers are somewhat smaller in physical size than the larger and more expensive transducers. This gives them a slight edge in S/N ratio as the field around a current carrying wire is greatest very near to the wire.
The present invention provides a current sensor for an electrical device which uses a plurality of current transducers in an array to provide a current measurement with lower noise levels and lower magnetic susceptibility. The orientation and number of the current transducers in the array affects the S/N ratio of the sensor. The sensor includes a housing, a conductor routed through the housing, an array of current transducers coupled to the conductor, and a current level signal generated by the current transducers. The conductor is adapted to be connected in a current path of the electrical device. Each of the current transducers has an output lead which carries the current level signal from the transducer. The output leads are electrically connected to a printed circuit board. A connector is electrically connected to the printed circuit board through a cable. A plurality of Faraday shields are disposed adjacent the current transducers to attenuate the field from the operating environment. The array of current transducers can be a single layer array, a multiple layer array, or a planar array.